Regarding an optical element provided in the vicinity of an image sensor, there are known, for example, technologies disclosed in PTL 1 and PTL 2.
PTL 1 discloses a parallax image input apparatus adapted to selectively image information that passed through different positions in an image formation means that forms images of an outside, to convert the image information to image data strings, and to store the converted image data strings. According to the technology of PTL 1, a plurality of images from different viewpoints can be captured.
PTL 2 discloses a digital camera adapted to deflect incident luminous flux into two directions by a deflection means, to detect two images by the luminous flux with image sensors, to calculate, based on an image shift amount obtained from detected image positions, a focus adjustment state of an object image adjusted by an imaging lens, and to move the imaging lens to a focus position. According to the technology of PTL 2, focus adjustment may quickly be carried out by one image detecting operation.
However, in the technology of PTL 1, a pupil is spatially divided by microlenses and light receiving cells for the luminous flux incident into a general image formation lens, and only a single focus is present. Therefore, a focus adjustment mechanism is needed in order to acquire images with different focal lengths. The focus adjustment mechanism is also needed in the case of PTL 2.
If the focus adjustment mechanism is incorporated in the imaging lens in order to take images with different focal lengths, a problem of upsizing of a drive unit arises.
As a solution to the problem, PTL 3 discloses an imaging optical system, in which one or two lens surfaces in an imaging lens system have a difference in curvature between in an inner region and in an outer region that are coaxial with an optical axis, and which satisfies 0.3≦f2/f1≦0.9 wherein f1 represents a focal length of the entire system with the curvature of the outer region and f2 represents a focal length of the entire system with the curvature of the inner region.
Moreover, PTL 4 discloses a technology for switching, with use of fluid prisms, the focus of a multifocal lens which has two lens regions different in focal length.
According to the technologies in PTL 3 and PTL 4, images with different focal lengths can be taken without the use of the focus adjustment mechanism.